There are several and known techniques of estimation of transmission channels in a multi-antenna system comprising several transmit antennas.
Most of these estimation techniques are limited to the application of a space-time encoding or space-frequency encoding in OFDM-type multi-carrier systems.
Thus, the first systems proposed all used orthogonal space-time block codes.
Alamouti in “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Selected Areas in Communications, pp. 311-335, vol. 6, 1998, presented the first system using an orthogonal space-time block code of rate 1 (where rate is defined as the ratio between the number N of symbols transmitted and the number L of symbol times during which they are transmitted) for two transmit antennas.
A major drawback of Alamouti's orthogonal space-time codes is that they are limited to two-transmit-antenna systems and that it is not possible to extend their use directly to a system with more than two transmit antennas while keeping an unitary rate.
Tarokh and al. (“Space-time Block Codes from Orthogonal Designs”, IEEE Transactions on Information Theory, 1999, 45, (5), pp. 1456-1467) then extended the orthogonal space-time block codes to systems comprising three or four transmit antennas. However, the rates R=N/L obtained were only ½ or ¾.
One drawback of Tarokh's orthogonal space-time codes therefore is that, although they are adapted to systems implementing a greater number of transmit antennas (three or four antennas), they have a rate of less than 1.
Barhumi and al. in “Pilot Tone-based Channel estimation for OFDM Systemes with Transmitter Diversity in Mobile Wireless Channels” then proposed a channel estimation technique for multi-antenna OFDM (SISO-OFDM or MIMO-OFDM) systems relying on a classic OFDM channel estimation system, implementing an extinction of certain carriers. However, one drawback of this estimation technique in a MIMO system is that the insertion of reference symbols generally causes major loss of spectral efficiency whenever, for each transmit antenna, a reference symbol is transmitted on a reference carrier at a given point in time while no data whatsoever is transmitted on the other carrier or carriers so as not to disturb the estimation of the transmission channel.
Other research was subsequently conducted by Fragouli and al. in “Training Based Channel Estimation for Multiple-Antenna Broadband Transmissions” on the learning sequences that can be used for channel estimation for multi-antenna systems.
Subsequently, Stirling-Gallacher and al. (“Improving performance of coherent coded OFDM systems using space time transmit diversity”, Electronics Letters, Vol. 37 N, March 2001, “Practical Pilot Patterns coherent coded OFDM systems using space time transmit diversity”, European Wireless 2002 conference, 25-28 Feb. 2002, Florence) envisaged a channel estimation technique for MIMO-OFDM systems, restricted to two-transmit-antenna systems using orthogonal space-time codes of the Alamouti or Tarokh type.
One drawback of this estimation technique is that the number of transmit antennas of the transmission system is limited by the use of prior-art orthogonal space-time block codes.
Thus, according to the prior-art techniques, there are no complex orthogonal unit-rate codes for systems having more than two transmit antennas. This diminishes spectral efficiency.